Transmission medium testing apparatus and method

ABSTRACT

The invention provides a method for testing a transmission medium used in a full-duplex communication system comprising an endpoint that comprises a transmitting end (TX) and a receiving end (RX); the method comprises the steps of: first, transmitting a transmitted signal which comprises a test signal sequence with a high auto-correlation characteristic; then, receiving a received signal, and performing a correlation operation on the test signal and the received signal; finally, according to the result of the correlation operation, determining the impedance matching condition of the transmission medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication system, especially to atransmission medium testing apparatus and method applying to acommunication system.

2. Description of the Prior Art

In communication system, a transmission medium is used to transmitsignals from one endpoint to a remote endpoint. The condition of thetransmission medium, such as the length of the cable, whether a shortcircuit, an open circuit, or an impedance-mismatching point in thecable, all significantly influences the quality of the transmittedsignal. Therefore, transmission medium testing apparatuses and methodsare used to detect the condition of the transmission medium in thecommunication system.

Please refer to FIG. 1A, FIG. 1B, and FIG. 1C; FIG. 1A to 1C show theschematic diagrams of the cable testing method in the prior art. Themethod for testing a cable in the prior art is to transmit a pulse wavefrom endpoint A; if the cable is in good condition, the pulsewave/transmitting wave will be transmitted from endpoint A to endpointB, as shown in FIG. 1A. If the cable has an impedance-mismatching pointC, such as short circuit or open circuit, the pulse wave/transmittingwave produces a reflected wave from endpoint C back to endpoint A, asshown in FIG. 1B. The time spent by the detected reflected wave to reachendpoint A depends on the location of the impedance-mismatching point C,and the impedance mismatching condition of the impedance-mismatchingpoint C depends on the characteristics of the reflected wave (such as:the phase). Therefore, the method for testing a cable in the prior artis to transmit a signal from endpoint A, and the time spent by thedetected reflected wave to reach endpoint A and the dimension of thereflected wave are used to determine the location and the condition ofthe impedance-mismatching point on the cable.

The cable testing method in the prior art described above can only beapplied in a half-duplex communication system. If a full-duplex networksystem is used, such as 1000 Mbps Ethernet, when endpoint A transmits apulse wave, the endpoint B also transmits a signal to endpoint A.Therefore, while cable testing is being performed, the endpoint A willreceive both the reflected wave caused by the impedance-mismatchingpoint C and the transmitting wave transmitted from endpoint B, as shownin FIG. 1C. Furthermore, when the transmitting end (Tx) of thefull-duplex network system transmits signals, the transmitted signalwill generate an echo effect at the receiving end (Rx). Therefore, thefull-duplex network system cannot determine the location and thecondition of the impedance-mismatching point on the cable from thesignal received at endpoint A. Consequently, the method using thereflected wave to test cable in the prior art cannot be used in thefull-duplex network system, meaning the application of the prior art islimited.

Therefore, the main objective of the present invention is to provide atransmission medium testing apparatus and method to solve the problemsdescribed above.

SUMMARY OF THE INVENTION

The main objective of the present invention is to provide a transmissionmedium testing apparatus and method used in a communication system tosolve the problem of the prior art.

According to the objective described above, the present inventionprovides a method for testing a transmission medium in a communicationsystem which comprises an endpoint, wherein the endpoint comprises atransmitting end (TX) and a receiving end (RX). A transmitted signalcomprising a test signal sequence with high auto-correlation is firsttransmitted. Then a received signal is received and a correlationoperation is performed on the transmitted signal and the receivedsignal. The impedance matching condition of the transmission medium isdetermined according to the result of the correlation operation at last.

According to the objection described above, this invention provides anapparatus for testing a transmission medium used in a communicationsystem which comprises an endpoint coupled to the transmission medium;the endpoint comprises a transmitting end (TX) for transmitting atransmitted signal and a receiving end (RX) for receiving a receivedsignal. The apparatus comprises a test-signal-sequence-transmittingcircuit, a correlation operation circuit, and an impedance-matchingdetermining circuit. The test-signal-sequence-transmitting circuit isdisposed at the transmitting end for transmitting a test signal sequencewith high auto-correlation to the transmission medium. The correlationoperation circuit is disposed at the receiving end for performing acorrelation operation on the received signal and the test signalsequence. The impedance-matching determining circuit determines theimpedance matching condition of the transmission medium according to theresult of the correlation operation.

The advantage and spirit of the invention may be understood by thefollowing recitations together with the appended drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1A to 1C show the schematic diagrams of the cable testing method inthe prior art.

FIG. 2 shows the flow chart of an embodiment of the transmission mediumtesting method for a communication system.

FIG. 3 shows the block diagram of an embodiment of the transmissionmedium testing apparatus for a communication system.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 2, which shows the flow chart of an embodiment ofthe transmission medium testing method for a communication system. Firststep 202 is performed to transmit a test signal sequence which compilesa plurality of signals from endpoint A, wherein the test signal sequencehas a high auto-correlation characteristic. In other words, when acorrelation operation is performed on the test signal sequence and othersignal sequences, such as when performing a product-of-sum operation,the result is obviously different from the result of performing acorrelation operation on the test signal sequence and itself. Besides,if the correlation operation is performed on two different test signalsequences, the phase difference of the two test signal sequences willaffect the result of the operation.

In this embodiment, the test signal sequence is a Pseudo-noise sequencewith a period of 2^(n-1) (i.e. comprising 2^(n-1) signals). When theproduct-of-sum operation is performed on the Pseudo-noise sequence andother test signal sequences, the result is obviously different from theresult of performing the product-of-sum operation on two Pseudo-noisesequences.

Besides, if there is a phase difference between the two Pseudo-noisesequences, the result of performing the product-of-sum operation is afirst constant (e.g. −1), but if two Pseudo-noise sequences with nophase difference are used, the result of the product-of-sum operation isa second constant (e.g. 2^(n-1)), which is obviously different from thefirst constant. For the description of the auto-correlation of therelated characteristics, correlation operations, and Pseudo-noisesequences mentioned above, please refer to Simon Haykin, “CommunicationSystem (3^(rd). ed. 1996)” published by John Wiley & Sons, Inc., pp.578-589.

Then, step 204 is performed, in which the energy level of the echosignal in the first time section is detected. Taking the 1000 MbpsEthernet as an example, which is a full-duplex communication system,both the transmitting end (TX) and the receiving end (RX) employ thesame pair of cable for transmitting/receiving signal. In a very shorttime after the transmitting end transmits the signal, the receiving endalso receives an echo signal caused by the transmitted signal itself.Therefore, when there is an impedance-mismatching point very close tothe endpoint A of the cable, the receiving end receives a mixture of theecho signal and the reflected signal of the transmitting signal. In thisembodiment, detect the energy level of the echo signal within the firsttime section after the transmitting end transmits the test signalsequence.

If the energy level of the echo signal is larger than a threshold (Step206), it shows that the signal received at the receiving end is not onlythe echo signal, but also the reflected signal of the transmittingsignal.

Therefore, step 208 is performed to confirm that there is at least oneimpedance-mismatching point located very close to endpoint A on thecable. The method for detecting the energy level of a signal by usingcircuits is well understood by people skilled in this art. For example,an adder or an integrator can both be employed for detecting the energylevel of the echo signal. Then, a comparator is employed to compare theenergy level with the threshold, and the steps mentioned above can beperformed; however, this invention is not limited to this. Furthermore,another method to perform this step is to process the echo cancellationby the echo cancellation circuit disposed at the receiving end and thento detect the echo residual. When the echo residual is larger than thethreshold, it shows that the echo signal comprises the reflected signal.

Next, step 210 is performed. By the end of the first time section, thereflected wave of the test signal sequence and the echo effect can bediscriminated clearly. At this time, the correlation operation isperformed on the signal received at endpoint A with the originaltransmitted signal. From the previous description, it is already knownthat in the full-duplex communication system, the signal received at thereceiving end is a mixture of the reflected signal of the transmittingsignal and the transmitting signal transmitted from a distant endpoint.Furthermore, the Pseudo-noise sequence employed in this invention hasthe high auto-correlation characteristic as in the above description.

Therefore, according to the result of performing the correlationoperation on the signal received at endpoint A and the originaltransmitted signal, the signal received at endpoint A can be determinedwhether it is a pure transmitted signal transmitted from the endpointfrom a long distance, or it comprises the reflected signal of thePseudo-noise sequence (step 212).

The correlation operation method of this embodiment sequentially adjuststhe phase of the received signal at endpoint A, and then performs theproduct-of-sum operation on the received signal and the Pseudo-noisesequence (step 214).

In the above description, when the received signal comprises thereflected wave of the transmitted signal, adjust the phase of thereceived signal to eliminate the phase difference between the reflectedwave of the transmitted signal included in the received signal and thePseudo-noise sequence transmitted for the Pseudo-noise sequence has highauto-correlation. There is obvious difference between the result of theproduct-of-sum operation and of other conditions (for example, there isno reflected wave of the transmitted signal in the received signal, orthere is phase difference between the reflected wave of the transmittedsignal and the Pseudo-noise sequence). In a preferred embodiment, bysequentially adjusting the phase of the received signal at endpoint Aand performing product-of-sum operation on the Pseudo-noise sequence andaccording to the result of the product-of-sum operation, severalconditions can be known: 1. Whether the received signal of the endpointA has the reflected wave of the transmitted signal. 2. If yes, what thedimension of the phase difference between the reflected wave of thetransmitted signal and Pseudo-noise sequence is.

To test a transmission medium by using the test signal sequence withhigh auto-correlation, such as a Pseudo-noise sequence, whether areceived signal comprises a reflected wave of a transmitted signal canbe determined by performing correlation operation on the received signaland the transmitted signal. Therefore, this invention can be applied toa communication system, such as 1000 Mbps Ethernet, but not limited tothat at the same time.

In addition, under the condition that the transmitting velocity (V) ofthe signal on the cable is already known, the time difference (T)between the endpoint A sending the signal to the cable and the endpointA receiving the reflected waves of the transmitted signal can bedetected, and the position of the impedance-mismatching point on thecable can then be determined (V×T/2). However, the above method is toosimplified for that practical communication system and the result oftests can be affected by many non-ideal situations. To test atransmission medium, ex: cable, by using the test signal sequence withhigh auto-correlation, such as a Pseudo-noise sequence, the phase of thereceived signal can then be adjusted and the correlation operation canbe performed on the adjusted signal and the original transmitted signal,thus the phase difference (p) between the original transmitted signaland the reflected wave of the transmitted signal can be obtained.Because the transmitting time of each symbol of the transmitted testingsignal sequence is a constant (t) and the transmitting velocity (V) ofthe signal on the cable is known, the position of theimpedance-mismatching point on the cable can be more preciselydetermined (V×p×t/2).

Step 216 is performed then. In practical network, the impedance of eachlocation is different from the ideal value. Accordingly, an impedancematching function is set to express the level of tolerance of theimpedance mismatch in practical network. When the signal is transmittedon the cable, the impedance matching function is determined by thereflective coefficient (Γ), of which its value is related to thedifference between difference between the real value of theimpedance-mismatching point (Zp) and the ideal impedance (Zi). Therelation is shown as following formula:$\Gamma = \frac{{Zp} - {Zi}}{{Zp} + {Zi}}$

Therefore, according to the impedance mismatch in the communicationsystem, which is the tolerance of the difference between the practicalimpedance value and the ideal impedance value, the threshold value ofthe impedance matching function is determined. When the signal istransmitted on the cable, the signal attenuates with the increasingdistance of the transmission. Therefore, when the impedance matchingfunction corresponding to each endpoint on the cable is beingdetermined, the propagation distance of signals transmitted fromendpoint A to each endpoint of the cable and the signal attenuationdegree also need to be taken into consideration. Generally speaking, thesignal attenuation degree of the transmitted signal is an exponentialfunction of the propagation distance (exp(−α×L), wherein α is anattenuation constant and L is the propagation distance). Therefore, foreach endpoint on the cable, the impedance matching function is not aconstant but a function value which relates to the propagation distance.In this embodiment, the function is an exponential function.

Since the reflected wave and the transmitted signal may be in-phase orphase-inverse, the impedance matching function comprises in-phase andphase-inverse. When step 216 is performed, and endpoint A detects thereceiving of the reflected wave of the transmitting signal, the positionof the impedance-mismatching point corresponding to the reflected waveis first determined according to the method of the embodiment describedabove.

Then, the energy level of the reflected wave is calculated and is thencompared with the impedance matching function corresponding to thelocation of the impedance-mismatching point. The comparison helps tojudge whether the impedance mismatching degree of theimpedance-mismatching point of the cable is larger than the level oftolerance of the communication system.

If it is larger than the level of tolerance of the communication system,step 218 is performed to determine whether there is at least oneimpedance-mismatching point on the cable. With the method of calculationdescribed above, the location of the impedance-mismatching point on thecable is determined. The theory and the method to detect and calculatethe energy of the reflected wave is the same as the theory and methodmentioned before. Please refer to the description above, and it will notbe described in detail again here.

FIG. 3 shows the block diagram of an embodiment of the transmissionmedium testing apparatus for a communication system. In FIG. 3,test-signal-sequence-transmitting circuit 302 is used for sending out atest signal sequence with high auto-correlation to the transmissionmedium. The second energy-detecting device 304 is used for detecting theenergy level of an echo signal corresponding to a transmitted signal inthe first time period. The correlation operation circuit 306 is used forperforming a correlation operation on the received signal and the testsignal sequence in the second time period, and a first energy-detectingdevice 308 is used for detecting the energy level of the reflectedsignal. The impedance-matching-determining circuit 310 is used forjudging whether there is at least one impedance matching point within ashort distance on the cable according to the test result of the secondenergy-detecting device 304; it is also used for judging, according tothe result of the correlation operation of the correlation operationcircuit 306, whether the received signal includes the reflected wave ofthe transmitted signal and for determining the location of theimpedance-mismatching point corresponding to the reflected wave.Furthermore, according to the detecting result of the firstenergy-detecting device 308, the impedance-matching-determining circuit310 judges whether the impedance mismatching condition of theimpedance-mismatching point can be tolerated by the communicationsystem. It is necessary to notice that the transmission medium testingapparatus for a communication system shown in FIG. 3 is only oneembodiment of the testing method described above, and this invention isnot limited to this embodiment.

The cable described above is not limited to various kinds of cables inthe prior art. Any transmission medium used in a communication systemand transmitting/receiving signals between endpoints, such as twistedwire pairs, can adapt the present invention for testing. Moreover, themethod described above cannot only test impedance-mismatching points butalso the length of the cable. Persons skilled in the art can easily putthe method into practice according to the description above, and detailswill not be described again here.

With the example and explanations above, the features and spirits of theinvention will be hopefully well described. Those skilled in the artwill readily observe that numerous modifications and alterations of thedevice may be made while retaining the teaching of the invention.Accordingly, the above disclosure should be construed as limited only bythe metes and bounds of the appended claims.

1. A method for testing a transmission medium used in a communicationsystem comprising an endpoint which comprises a transmitting end (TX)and a receiving end (RX), the method comprising: transmitting atransmitted signal comprising a test signal sequence withauto-correlation; receiving a received signal and performing acorrelation operation on the test signal and the received signal; anddetermining the matching condition of an impedance of the transmissionmedium according to the result of the correlation operation.
 2. Themethod of claim 1, wherein the test signal sequence is a pseudo-noisesequence.
 3. The method of claim 1, wherein the received signalcomprises a reflected signal of the transmitted signal, the result ofthe correlation operation relates to a phase difference between thereflected signal and the transmitted signal.
 4. The method of claim 3,wherein the correlation operation is a product-of-sum operation.
 5. Themethod of claim 3, wherein the correlation operation is performed byadjusting the phase of the received signal and performing aproduct-of-sum operation on the adjusted received signal and thetransmitted signal.
 6. The method of claim 3, wherein the condition ofthe transmission medium is determined according to the phase differencebetween the reflected signal and the transmitted signal.
 7. The methodof claim 1, wherein the test signal sequence comprises a plurality ofsymbols, each of which is transmitted in a certain transmissionvelocity, and a time interval exists between the transmission time ofadjacent symbols; the position of at least one impedance-mismatchingpoint on the transmission medium can be determined according to thetransmission signal, the transmission time, and the phase differencebetween the reflected signal and the transmitted signal.
 8. The methodof claim 1, further comprising the steps of: detecting an energy levelof a reflected signal of the transmitted signal; and determining thematching condition of the impedance of the transmission medium accordingto the result of detection the energy level of the reflected signal. 9.The method of claim 8, wherein the step of determining the matchingcondition of the transmission medium comprises: determining a positionon the transmission medium according to the reflected signal, whereinthe position corresponds to the reflected signal; determining animpedance matching function corresponding to the function value of theposition; and comparing the result of the detection of the energy levelof the reflected signal with the value of the impedance matchingfunction to determine whether the position is an impedance-mismatchingpoint.
 10. The method of claim 9, wherein the impedance matchingfunction is determined according to a reflective coefficient, a signalattenuation degree, and a signal propagation distance, and the signalpropagation distance is the distance between the endpoint and theposition.
 11. The method of claim 10, wherein the impedance matchingfunction is an exponential function of the signal propagation distance.12. The method of claim 1, further comprising the steps of: detectingthe energy level of an echo signal corresponding to the transmittedsignal; and determining the matching condition of the impedance of thetransmission medium according to the result of detection of the energylevel of the echo signal.
 13. The method of claim 12, wherein the stepof determining the condition of the impedance of the transmission mediumcomprising: comparing the result of the detection of the energy level ofthe echo signal with a threshold; and if the result of the detection ofthe energy level of the echo signal is larger than the threshold,determining that the transmission medium thereon has at least oneimpedance-mismatching point in a short distance apart from the endpoint.14. The method of claim 1, wherein the communication system is a 1000Mbps Ethernet.
 15. An apparatus for testing a transmission medium usedin a communication system comprising an endpoint coupled to thetransmission medium, the endpoint comprising a transmitting end (TX) fortransmitting a transmitted signal and a receiving end (RX) for receivinga received signal, the apparatus comprising: a test signal generator,disposed at the TX, for generating a test signal with auto-correlationto the transmission medium; a correlation operating circuit, disposed atthe RX, for performing a correlation operation on the received signaland the test signal; and a decision circuit for determining thecondition the transmission medium according to the result of thecorrelation operation.
 16. The apparatus of claim 15, wherein thedecision circuit determines a position of at least oneimpedance-mismatching point on the transmission medium according to thephase difference between a reflected signal corresponding to the testsignal and the test signal.
 17. The apparatus of claim 15, wherein thereceived signal comprises a reflected signal of the transmitted signal.18. The apparatus of claim 15, further comprising a firstenergy-detecting circuit for detecting an energy level of a reflectedsignal of the transmitted signal.
 19. The apparatus of claim 15, furthercomprising a second energy-detecting circuit for detecting the energylevel of an echo signal corresponding to the transmitted signal.
 20. Theapparatus of claim 15, wherein the test signal is a pseudo-noisesequence.